Bone resorption is essential for all aspects of bone physiology: growth, remodeling, and repair. The most prevalent disease associated with an abnormality in bone resorption is osteoporosis, which is frequently a result of an excess bone resorption. The osteoclast is the primary bone-resorbing cells. Understanding the molecular mechanisms that regulate osteoclast formation and activity could provide insights into the pathophysiology of osteoporosis, and also better use of the currently available anti-resorptive drugs for bone wasting diseases. This project seeks to define the functional role of a new osteoclastic gene, osteoactivin (OA), in osteoclast formation and function and its potential molecular mechanism. OA is expressed in a wide variety of tissues and has been suggested to play a regulatory role in osteoblast differentiation and in endothelial cell adhesion. This proposal presented evidence that 1) OA is highly expressed in mature osteoclasts and its expression is upregulated during the differentiation of osteoclasts, 2) OA has a regulatory role in osteoclast formation, spreading, and activity, and 3) its molecular mechanism involves an interaction with integrin [unreadable]3 and/or [unreadable]1, presumably through its RGD motifs. Accordingly, this revised proposal tests two specific hypotheses: 1) OA has an important regulatory role in osteoclast formation and bone resorption activity in vivo and in vitro;and 2) OA regulates osteoclast formation and/or bone resorption activity in an integrin-dependent manner through its RGD motifs. The first hypothesis is addressed by two approaches: In the in vitro approach, primary murine osteoclast precursors will be used to determine if siRNA-mediated OA gene knockdown interferes with the RANKL-induced differentiation of osteoclasts as measured by osteoclast size, number of nuclei per osteoclast, migration, and bone resorption. The second in vivo approach is to generate transgenic mice with targeted overexpression of OA in cells of osteoclast lineage using the tartrate-resistant acid phosphatase C promoter. The in vivo function of OA on osteoclast formation and resorption will be determined by assessing the effects of targeted overexpression of OA on osteoclast formation and functions in the transgenic mice in vivo. The second hypothesis is tested by two strategies: the first strategy is to, with the use of preosteoclasts of OA transgenic mice, determine if siRNA-mediated knockdown of integrin [unreadable]3 and/or [unreadable]1 expression blocks the OA-induced increase in osteoclast formation and/or functions. The second strategy is to determine the effects of forced overexpression of RGD-mutated OA variants in murine osteoclast precursors on osteoclast formation, migration, and resorption, and compare the effects of forced overexpression of RGD- mutants in osteoblasts on osteoblast differentiation. If the hypotheses are correct, this work will provide insights into the functional role of this novel osteoclastic gene and define the interaction between OA and the integrin signaling in the regulation of osteoclast formation and activity. It may also provide a novel gene or pathway target for development of novel and effective treatment of osteoporosis and related bone-wasting diseases.